Network Working Group D. Farinacci
Internet-Draft D. Lewis
Intended status: Informational D. Meyer
Expires: October 25, 2012 cisco Systems
C. White
Logical Elegance, LLC.
April 23, 2012
LISP Mobile Nodedraft-meyer-lisp-mn-07
Abstract
This document describes how a lightweight version of LISP's ITR/ETR
functionality can be used to provide seamless mobility to a mobile
node. The LISP Mobile Node design described in this document uses
standard LISP functionality to provide scalable mobility for LISP
mobile nodes.
Status of this Memo
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This Internet-Draft will expire on October 25, 2012.
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Internet-Draft LISP Mobile Node April 20121. Introduction
The Locator/ID Separation Protocol (LISP) [LISP] specifies a design
and mechanism for replacing the addresses currently used in the
Internet with two separate name spaces: Endpoint Identifiers (EIDs),
used within sites, and Routing Locators (RLOCs), used by the transit
networks that make up the Internet infrastructure. To achieve this
separation, LISP defines protocol mechanisms for mapping from EIDs to
RLOCs. The mapping infrastructure is comprised of LISP Map-Servers
and Map-Resolvers [LISP-MS] and is tied together with LISP+ALT
[LISP-ALT].
This document specifies the behavior of a new LISP network element:
the LISP Mobile Node. The LISP Mobile Node implements a subset of
the standard Ingress Tunnel Router and Egress Tunnel Router
functionality [LISP]. Design goals for the LISP mobility design
include:
o Allowing TCP connections to stay alive while roaming.
o Allowing the mobile node to communicate with other mobile nodes
while either or both are roaming.
o Allowing the mobile node to multi-home (i.e., use multiple
interfaces concurrently).
o Allowing the mobile node to be a server. That is, any mobile node
or stationary node can find and connect to a mobile node as a
server.
o Providing shortest path bidirectional data paths between a mobile
node and any other stationary or mobile node.
o Not requiring fine-grained routes in the core network to support
mobility.
o Not requiring a home-agent, foreign agent or other data plane
network elements to support mobility. Note since the LISP mobile
node design does not require these data plane elements, there is
no triangle routing of data packets as is found in Mobile IP
[RFC3344].
o Not requiring new IPv6 extension headers to avoid triangle routing
[RFC3775].
The LISP Mobile Node design requires the use of the LISP Map-Server
[LISP-ALT] and LISP Interworking [LISP-INTERWORK] technology to allow
a LISP mobile node to roam and to be discovered in an efficient and
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scalable manner. The use of Map-Server technology is discussed
further in Section 5.
The protocol mechanisms described in this document apply those cases
in which a node's IP address changes frequently. For example, when a
mobile node roams, it is typically assigned a new IP address.
Similarly, a broadband subscriber may have its address change
frequently; as such, a broadband subscriber can use the LISP Mobile
Node mechanisms defined in this specification.
The remainder of this document is organized as follows: Section 2
defines the terms used in this document. Section 3 provides a
overview of salient features of the LISP Mobile Node design, and
Section 4 describes design requirements for a LISP Mobile Node.
Section 5 provides the detail of LISP Mobile Node data and control
plane operation, and Section 6 discusses options for updating remote
caches in the presence of unidirectional traffic flows. Section 7
specifies how the LISP Mobile Node protocol operates. Section 8
specifies multicast operation for LISP mobile nodes. Section 9 and
Section 12 outline other considerations for the LISP-MN design and
implementation. Finally, Section 13 outlines the security
considerations for a LISP mobile node.
2. Definition of Terms
This section defines the terms used in this document.
Stationary Node (SN): A non-mobile node who's IP address changes
infrequently. That is, its IP address does not change as
frequently as a fast roaming mobile hand-set or a broadband
connection and therefore the EID to RLOC mapping is relatively
static.
Endpoint ID (EID): This is the traditional LISP EID [LISP], and is
the address that a LISP mobile node uses as its address for
transport connections. A LISP mobile node never changes its EID,
which is typically a /32 or /128 prefix and is assigned to a
loopback interface. Note that the mobile node can have multiple
EIDs, and these EIDs can be from different address families.
Routing Locator (RLOC): This is the traditional LISP RLOC, and is in
general a routable address that can be used to reach a mobile
node. Note that there are cases in which an mobile node may
receive an address that it thinks is an RLOC (perhaps via DHCP)
which is either an EID or an RFC 1918 address [RFC1918]. This
could happen if, for example, if the mobile node roams into a LISP
domain or a domain behind a Network Address Translator (NAT)) See
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Internet-Draft LISP Mobile Node April 2012Section 10 for more details.
Ingress Tunnel Router (ITR): An ITR is a router that accepts an IP
packet with a single IP header (more precisely, an IP packet that
does not contain a LISP header). The router treats this "inner"
IP destination address as an EID and performs an EID-to-RLOC
mapping lookup. The router then prepends an "outer" IP header
with one of its globally routable RLOCs in the source address
field and the result of the mapping lookup in the destination
address field. Note that this destination RLOC may be an
intermediate, proxy device that has better knowledge of the EID-
to-RLOC mapping closer to the destination EID. In general, an ITR
receives IP packets from site end-systems on one side and sends
LISP-encapsulated IP packets toward the Internet on the other
side. A LISP mobile node, however, when acting as an ITR LISP
encapsulates all packet that it originates.
Egress Tunnel Router (ETR): An ETR is a router that accepts an IP
packet where the destination address in the "outer" IP header is
one of its own RLOCs. The router strips the "outer" header and
forwards the packet based on the next IP header found. In
general, an ETR receives LISP-encapsulated IP packets from the
Internet on one side and sends decapsulated IP packets to site
end-systems on the other side. A LISP mobile node, when acting as
an ETR, decapsulates packets that are then typically processed by
the mobile node.
Proxy Ingress Tunnel Router (PITR): PITRs are used to provide
interconnectivity between sites that use LISP EIDs and those that
do not. They act as a gateway between the Legacy Internet and the
LISP enabled Network. A given PITR advertises one or more highly
aggregated EID prefixes into the public Internet and acts as the
ITR for traffic received from the public Internet. Proxy Ingress
Tunnel Routers are described in [LISP-INTERWORK].
Proxy Egress Tunnel Router (PETR): An infrastructure element used to
decapsulate packets sent from mobile nodes to non-LISP sites.
Proxy Egress Tunnel Routers are described in [LISP-INTERWORK].
LISP Mobile Node (LISP-MN): A LISP capable fast roaming mobile hand-
set.
Map-cache: A data structure which contains an EID-prefix, its
associated RLOCs, and the associated policy. Map-caches are
typically found in ITRs and PITRs.
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Negative Map-Reply: A Negative Map-Reply is a Map-Reply that
contains a coarsely aggregated non-LISP prefix. Negative Map-
Replies are typically generated by Map-Resolvers, and are used to
inform an ITR (mobile or stationary) that a site is not a LISP
site. A LISP mobile node encapsulate packets to destinations
covered by the negative Map-Reply are encapsulated to a PETR.
Roaming Event: A Roaming Event occurs when there is a change in a
LISP mobile node's RLOC set.
3. Design Overview
The LISP-MN design described in this document uses the Map-Server/
Map-Resolver service interface in conjunction with a light-weight
ITR/ETR implementation in the LISP-MN to provide scalable fast
mobility. The LISP-MN control-plane uses a Map-Server as an anchor
point, which provides control-plane scalability. In addition, the
LISP-MN data-plane takes advantage of shortest path routing and
therefore does not increase packet delivery latency.
4. Design Requirements
This section outlines the design requirements for a LISP-MN, and is
divided into User Requirements (Section 4.1) and Network Requirements
(Section 4.2).
4.1. User Requirements
This section describes the user-level functionality provided by a
LISP-MN.
Transport Connection Survivability: The LISP-MN design must allow a
LISP-MN to roam while keeping transport connections alive.
Simultaneous Roaming: The LISP-MN design must allow a LISP-MN to
talk to another LISP-MN while both are roaming.
Multihoming: The LISP-MN design must allow for simultaneous use of
multiple Internet connections by a LISP-MN. In addition, the
design must allow for the LISP mobile node to specify ingress
traffic engineering policies as documented in [LISP]. That is,
the LISP-MN must be able to specify both active/active and active/
passive policies for ingress traffic.
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Shortest Path Data Plane: The LISP-MN design must allow for shortest
path bidirectional traffic between a LISP-MN and a stationary
node, and between a LISP-MN and another LISP-MN (i.e., without
triangle routing in the data path). This provides a low-latency
data path between the LISP-MN and the nodes that it is
communicating with.
4.2. Network Requirements
This section describes the network functionality that the LISP-MN
design provides to a LISP-MN.
Routing System Scalability: The LISP-MN design must not require
injection of fine-grained routes into the core network.
Mapping System Scalability: The LISP-MN design must not require
additional state in the mapping system. In particular, any
mapping state required to support LISP mobility must BE confined
to the LISP-MN's Map-Server and the ITRs which are talking to the
LISP-MN.
Component Reuse: The LISP-MN design must use existing LISP
infrastructure components. These include map server, map
resolver, and interworking infrastructure components.
Home Agent/Foreign Agent: The LISP-MN design must not require the
use of home-agent or foreign-agent infrastructure components
[RFC3344].
Readdressing: The LISP-MN design must not require TCP connections to
be reset when the mobile node roams. In particular, since the IP
address associated with a transport connection will not change as
the mobile node roams, TCP connections will not reset.
5. LISP Mobile Node Operation
The LISP-MN design is built from three existing LISP components: A
lightweight LISP implementation that runs in an LISP-MN, and the
existing Map-Server [LISP-MS] and Interworking [LISP-INTERWORK]
infrastructures. A LISP mobile node typically sends and receives
LISP encapsulated packets (exceptions include management protocols
such as DHCP).
The LISP-MN design makes a single mobile node look like a LISP site
as described in in [LISP] by implementing ITR and ETR functionality.
Note that one subtle difference between standard ITR behavior and
LISP-MN is that the LISP-MN encapsulates all non-local, non-LISP site
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destined outgoing packets to a PETR.
When a LISP-MN roams onto a new network, it receives a new RLOC.
Since the LISP-MN is the authoritative ETR for its EID-prefix, it
must Map-Register it's updated RLOC set. New sessions can be
established as soon as the registration process completes. Sessions
that are encapsulating to RLOCs that did not change during the
roaming event are not affected by the roaming event (or subsequent
mapping update). However, the LISP-MN must update the ITRs and PITRs
that have cached a previous mapping. It does this using the
techniques described in Section 6.
5.1. Addressing Architecture
A LISP-MN is typically provisioned with one or more EIDs that it uses
for all transport connections. LISP-MN EIDs are provisioned from
blocks reserved from mobile nodes much the way mobile phone numbers
are provisioned today (such that they do not overlap with the EID
space of any enterprise). These EIDs can be either IPv4 or IPv6
addresses. For example, one EID might be for a public network while
another might be for a private network; in this case the "public" EID
will be associated with RLOCs from the public Internet, while the
"private" EID will be associated with private RLOCs. It is
anticipated that these EIDs will change infrequently if at all, since
the assignment of a LISP-MN's EID is envisioned to be a subscription
time event. The key point here is that the relatively fixed EID
allows the LISP-MN's transport connections to survive roaming events.
In particular, while the LISP-MN's EIDs are fixed during roaming
events, the LISP-MN's RLOC set will change. The RLOC set may be
comprised of both IPv4 or IPv6 addresses.
A LISP-MN is also provisioned with the address of a Map-Server and a
corresponding authentication key. Like the LISP-MN's EID, both the
Map-Server address and authentication key change very infrequently
(again, these are anticipated to be subscription time parameters).
Since the LISP LISP-MN's Map-Server is configured to advertise an
aggregated EID-prefix that covers the LISP-MN's EID, changes to the
LISP-MN's mapping are not propagated further into the mapping system
[LISP-ALT]. It is this property that provides for scalable fast
mobility.
A LISP-MN is also be provisioned with the address of a Map-Resolver.
A LISP-MN may also learn the address of a Map-Resolver though a
dynamic protocol such as DHCP [RFC2131].
Finally, note that if, for some reason, a LISP-MN's EID is re-
provisioned, the LISP-MN's Map-Server address may also have to change
in order to keep LISP-MN's EID within the aggregate advertised by the
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Map-Server (this is discussed in greater detail in Section 5.2).
5.2. Control Plane Operation
A roaming event occurs when the LISP-MN receives a new RLOC. Because
the new address is a new RLOC from the LISP-MN's perspective, it must
update its EID-to-RLOC mapping with its Map-Server; it does this
using the Map-Register mechanism described in [LISP].
A LISP-MN may want the Map-Server to respond on its behalf for a
variety of reasons, including minimizing control traffic on radio
links and minimizing battery utilization. A LISP-MN may instruct its
Map-Server to proxy respond to Map-Requests by setting the Proxy-Map-
Reply bit in the Map-Register message [LISP]. In this case the Map-
Server responds with a non-authoritative Map-Reply so that an ITR or
PITR will know that the ETR didn't directly respond. A Map-Server
will proxy reply only for "registered" EID-prefixes using the
registered EID-prefix mask-length in proxy replies.
Because the LISP-MN's Map-Server is pre-configured to advertise an
aggregate covering the LISP-MN's EID prefix, the database mapping
change associated with the roaming event is confined to the Map-
Server and those ITRs and PITRs that may have cached the previous
mapping.
5.3. Data Plane Operation
A key feature of LISP-MN control-plane design is the use of the Map-
Server as an anchor point; this allows control of the scope to which
changes to the mapping system must be propagated during roaming
events.
On the other hand, the LISP-MN data-plane design does not rely on
additional LISP infrastructure for communication between LISP nodes
(mobile or stationary). Data packets take the shortest path to and
from the LISP-MN to other LISP nodes; as noted above, low latency
shortest paths in the data-plane is an important goal for the LISP-MN
design (and is important for delay-sensitive applications like gaming
and voice-over-IP). Note that a LISP-MN will need additional
interworking infrastructure when talking to non-LISP sites
[LISP-INTERWORK]; this is consistent with the design of any host at a
LISP site which talks to a host at a non-LISP site.
In general, the LISP-MN data-plane operates in the same manner as the
standard LISP data-plane with one exception: packets generated by a
LISP-MN which are not destined for the mapping system (i.e., those
sent to destination UDP port 4342) or the local network are LISP
encapsulated. Because data packets are always encapsulated to a
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RLOC, packets travel on the shortest path from LISP-MN to another
LISP stationary or LISP-MN. When the LISP mobile node is sending
packets to a stationary or LISP-MN in a non-LISP site, it sends LISP-
encapsulated packets to a PETR which then decapsulates the packet and
forwards it to its destination.
6. Updating Remote Caches
A LISP-MN has five mechanisms it can use to cause the mappings cached
in remote ITRs and PITRs to be refreshed:
Map Versioning: If Map Versioning [LISP-VERSIONING] is used, an ETR
can detect if an ITR is using the most recent database mapping.
In particular, when mobile node's ETR decapsulates a packet and
detects the Destination Map-Version Number is less than the
current version for its mapping, in invokes the SMR procedure
described in [LISP]. In general, SMRs are used to fix the out of
sync mapping while Map-Versioning is used to detect they are out
of sync. [LISP-VERSIONING] provides additional details of the Map
Versioning process.
Data Driven SMRs: An ETR may elect to send SMRs to those sites it
has been receiving encapsulated packets from. This will occur
when an ITR is sending to an old RLOC (for which there is one-to-
one mapping between EID-to-RLOC) and the ETR may not have had a
chance to send an SMR the ITR.
Setting Small TTL on Map Replies: The ETR (or Map Server) may set a
small Time to Live (TTL) on its mappings when responding to Map
Requests. The TTL value should be chosen such that changes in
mappings can be detected while minimizing control traffic. In
this case the ITR is a SN and the ETR is the MN.
Piggybacking Mapping Data: If an ITR and ETR are co-located, an ITR
may elect to send Map-Requests with piggybacked mapping data to
those sites in its map cache or to which it has recently
encapsulated data in order to inform the remote ITRs and PITRs of
the change.
Temporary PITR Caching: The ETR can keep a cache of PITRs that have
sent Map-Requests to it. The cache contains the RLOCs of the
PITRs so later when the locator-set of a LISP-MN changes, SMR
messages can be sent to all RLOCs in the PITR cache. This is an
example of a control-plane driven SMR procedure.
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Internet-Draft LISP Mobile Node April 20127. Protocol Operation
There are five distinct connectivity cases considered by the LISP-MN
design. The five mobility cases are:
LISP Mobile Node to a Stationary Node in a LISP Site.
LISP Mobile Node to a Non-LISP Site.
LISP Mobile Node to a LISP Mobile Node.
Non-LISP Site to a LISP Mobile Node.
LISP Site to a LISP Mobile Node.
The remainder of this section covers these cases in detail.
7.1. LISP Mobile Node to a Stationary Node in a LISP Site
After a roaming event, a LISP-MN must immediately register its new
EID-to-RLOC mapping with its configured Map-Server(s). This allows
LISP sites sending Map-Requests to the LISP-MN to receive the current
mapping. In addition, remote ITRs and PITRs may have cached mappings
that are no longer valid. These ITRs and PITRs must be informed that
the mapping has changed. See Section 6 for a discussion of methods
for updating remote caches.
7.1.1. Handling Unidirectional Traffic
A problem may arise when traffic is flowing unidirectionally between
LISP sites. This can arise in communication flows between PITRs and
LISP sites or when a site's ITRs and ETRs are not co-located. In
these cases, data-plane techniques such as Map-Versioning and Data-
Driven SMRs can't be used to update the remote caches.
For example, consider the unidirectional packet flow case depicted in
Figure 1. In this case X is a non-LISP enabled SN (i.e., connected
to the Internet) and Y is a LISP MN. Data traffic from X to Y will
flow through a PITR. When Y changes its mapping (for example, during
a mobility event), the PITR must update its mapping for Y. However,
since data traffic from Y to X is unidirectional and does not flow
though the PITR, it can not rely data traffic from Y to X to signal a
mapping change at Y. In this case, the Y must use one or more of the
techniques described in Section 6 to update the PITR's cache. Note
that if Y has only one RLOC, then the PITR has to know when to send a
Map-Request based on its existing state; thus it can only rely on the
TTL on the existing mapping.
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+-------------------------------------------+
| |
| | DP
v DP DP MQ |
X -----> Internet -----> PITR ------------> Y
^ LEDP |
| |
+-----------------+
MR
DP: Data Packet
LEDP: LISP Encapsulated Data Packet
MQ: Map Request
MR: Map Reply
Figure 1: Unidirectional Packet Flow
7.2. LISP Mobile Node to a Non-LISP Stationary Node
LISP-MNs use the LISP Interworking infrastructure (specifically a
PETR) to reach non-LISP sites. In general, the PETR will be co-
located with the LISP-MN's Map-Server. This ensures that the LISP
packets being decapsulated are from sources that have Map-Registered
to the Map-Server. Note that when a LISP-MN roams it continues to
uses its configured PETR and Map-Server which can have the effect of
adding stretch to packets sent from a LISP-MN to a non-LISP
destination.
7.3. LISP Mobile Node to LISP Mobile Node
LISP-MN to LISP-MN communication is an instance of LISP-to-LISP
communication with three sub-cases:
o Both LISP-MNs are stationary (Section 7.1).
o Only one LISP-MN is roaming (Section 7.3.1).
o Both LISP-MNs are roaming. The case is analogous to the case
described in Section 7.3.1.
7.3.1. One Mobile Node is Roaming
In this case, the roaming LISP-MN can find the stationary LISP-MN by
sending Map-Request for its EID-prefix. After receiving a Map-Reply,
the roaming LISP-MN can encapsulate data packets directly to the non-
roaming LISP-MN node.
The roaming LISP-MN, on the other hand, must update its Map-Server
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with the new mapping data as described in Section 7.1. It should
also use the cache management techniques described in Section 6 to
provide for timely updates of remote caches. Once the roaming
LISP-MN has updated its Map-Server, the non-roaming LISP-MN can
retrieve the new mapping data (if it hasn't already received an
updated mapping via one of the mechanisms described in Section 6) and
the stationary LISP-MN can encapsulate data directly to the roaming
LISP-MN.
7.4. Non-LISP Site to a LISP Mobile Node
When a stationary ITR is talking to a non-LISP site, it may forward
packets natively (unencapsulated) to the non-LISP site. This will
occur when the ITR has received a negative Map Reply for a prefix
covering the non-LISP site's address with the Natively-Forward action
bit set [LISP]. As a result, packets may be natively forwarded to
non-LISP sites by an ITR (the return path will through a PITR,
however, since the packet flow will be non-LISP site to LISP site).
A LISP-MN behaves differently when talking to non-LISP sites. In
particular, the LISP-MN always encapsulates packets to a PETR. The
PETR then decapsulates the packet and forwards it natively to its
destination. As in the stationary case, packets from the non-LISP
site host return to the LISP-MN through a PITR. Since traffic
forwarded through a PITR is unidirectional, a LISP-MN should use the
cache management techniques described in Section 7.1.1.
7.5. LISP Site to LISP Mobile Node
When a LISP-MN roams onto a new network, it needs to update the
caches in any ITRs that might have stale mappings. This is analogous
to the case in that a stationary LISP site is renumbered; in that
case ITRs that have cached the old mapping must be updated. This is
done using the techniques described in Section 6.
When a LISP router in a stationary site is performing both ITR and
ETR functions, a LISP-MN can update the stationary site's map-caches
using techniques described in Section 6. However, when the LISP
router in the stationary site is performing is only ITR
functionality, these techniques can not be used because the ITR is
not receiving data traffic from the LISP-MN. In this case, the
LISP-MN should use the technique described in Section 7.1.1. In
particular, a LISP-MN should set the TTL on the mappings in its Map-
Replies to be in 1-2 minute range.
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Internet-Draft LISP Mobile Node April 20128. Multicast and Mobility
Since a LISP-MN performs both ITR and ETR functionality, it should
also perform a lightweight version of multicast ITR/ETR functionality
described in [MLISP]. When a LISP-MN originates a multicast packet,
it will encapsulate the packet with a multicast header, where the
source address in the outer header is one of it's RLOC addresses and
the destination address in the outer header is the group address from
the inner header. The interfaces in which the encapsulated packet is
sent on is discussed below.
To not require PIM functionality in the LISP-MN as documented in
[MLISP], the LISP-MN resorts to using encapsulated IGMP for joining
groups and for determining which interfaces are used for packet
origination. When a LISP-MN joins a group, it obtains the map-cache
entry for the (S-EID,G) it is joining. It then builds a IGMP report
encoding (S-EID,G) and then LISP encapsulates it with UDP port 4341.
It selects an RLOC from the map-cache entry to send the encapsulated
IGMP Report.
When other LISP-MNs are joining an (S-EID,G) entry where the S-EID is
for a LISP-MN, the encapsulated IGMP Report will be received by the
LISP-MN multicast source. The LISP-MN multicast source will remember
the interfaces the encapsulated IGMP Report is received on and build
an outgoing interface list for it's own (S-EID,G) entry. If the list
is greater than one, then the LISP-MN is doing replication on the
source-based tree for which it is the root.
When other LISP routers are joining (S-EID,G), they are instructed to
send PIM encapsulated Join-Prune messages. However, to keep the
LISP-MN as simple as possible, the LISP-MN will not be able to
process encapsulated PIM Join-Prune messages. Because the map-cache
entry will have a MN-bit indicating the entry is for a LISP-MN, the
LISP router will send IGMP encapsulated IGMP Reports instead.
When the LISP-MN is sending a multicast packet, it can operate in two
modes, multicast-origination-mode or unicast-origination-mode. When
in multicast-origination-mode, the LISP-MN multicast-source can
encapsulate a multicast packet in another multicast packet, as
described above. When in unicast-origination-mode, the LISP-MN
multicast source encapsulates the multicast packet into a unicast
packet and sends a packet to each encapsulated IGMP Report sender.
These modes are provided depending on whether or not the mobile
node's network it is currently connected can support IP multicast.
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Internet-Draft LISP Mobile Node April 20129. RLOC Considerations
This section documents cases where the expected operation of the
LISP-MN design may require special treatment.
9.1. Mobile Node's RLOC is an EID
When a LISP-MN roams into a LISP site, the "RLOC" it is assigned may
be an address taken from the site's EID-prefix. In this case, the
LISP-MN will Map-Register a mapping from its statically assigned EID
to the "RLOC" it received from the site. This scenario creates
another level of indirection: the mapping from the LISP-MN's EID to a
site assigned EID. The mapping from the LISP-MN's EID to the site
assigned EID allow the LISP-MN to be reached by sending packets using
the mapping for the EID; packets are delivered to site's EIDs use the
same LISP infrastructure that all LISP hosts use to reach the site.
A packet egressing a LISP site destined for a LISP-MN that resides in
a LISP site will have three headers: an inner header that is built by
the host and is used by transport connections, a middle header that
is built by the site's ITR and is used by the destination's ETR to
find the current topological location of the LISP-MN, and an outer
header (also built by the site's ITR) that is used to forward packets
between the sites.
Consider a site A with EID-prefix 1.0.0.0/8 and RLOC A and a site B
with EID-prefix 2.0.0.0/8 and RLOC B. Suppose that a host S in site A
with EID 1.0.0.1 wants to talk to a LISP LISP-MN MN that has
registered a mapping from EID 240.0.0.1 to "RLOC" 2.0.0.2 (where
2.0.0.2 allocated from site B's EID prefix, 2.0.0.0/8 in this case).
This situation is depicted in Figure 2.
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2. The MN which is outside (call it Outside) sends a map request for
inside's EID (E) and receives D (plus its policy). Outside
realizes that D is an EID and sends a map request for D. This
will return the site's RLOCs (by its ETR). Call this R.
3. Outside then double encapsulates the outbound packet with the
inner destination being D and the outer destination being R.
4. The packet then finds its way to R, which strips the outer header
and the packet is routed to D in the domain to Inside. Inside
decapsulates the packet to serve the inner header to the
application.
Note that both D and R could be returned to Inside in one query, so
as not to incur the additional RTT.
10. LISP Mobile Nodes behind NAT Devices
When a LISP-MN resides behind a NAT device, it will be allocated a
private RLOC address. The private RLOC address is used as the source
address in the outer header for LISP encapsulated packets. The NAT
device will translate the source address and source UDP port in the
LISP encapsulated packet. The NAT device will keep this translated
state so when packets arrive from the public side of the NAT, they
can be translated back to the stored state. For remote LISP ITRs,
PITRs, and RTRs, will need to know the translated RLOC address and
port so they can encapsulate to the LISP-MN traversing the NAT
device.
Procedures a LISP-MN should follow when it resides behind a NAT, will
follow the LISP xTRs procedures in specification [LISP-NATT].
11. Mobility Example
This section provides an example of how the LISP-MN is integrated
into the base LISP Design [LISP].
11.1. Provisioning
The LISP-MN needs to be configured with the following information:
An EID, assigned to its loopback address
A key for map-registration
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An IP address of a Map-Resolver (this could be learned
dynamically)
An IP address of its Map-Server and Proxy ETR
11.2. Registration
After a LISP roams to a new network, it must immediately register its
new mapping this new RLOC (and associated priority/weight data) with
its Map-Server.
The LISP-MN may chose to set the 'proxy' bit in the map-register to
indicate that it desires its Map-Server to answer map-requests on its
behalf.
12. LISP Implementation in a Mobile Node
This section will describe a possible approach for developing a
lightweight LISP-MN implementation. A LISP-MN will implement a LISP
sub-layer inside of the IP layer of the protocol stack. The sub-
layer resides between the IP layer and the link-layer.
For outgoing unicast packets, once the header that contains EIDs is
built and right before an outgoing interface is chosen, a LISP header
is prepended to the outgoing packet. The source address is set to
the local RLOC address (obtained by DHCP perhaps) and the destination
address is set to the RLOC associated with the destination EID from
the IP layer. To obtain the RLOC for the EID, the LISP-MN maintains
a map-cache for destination sites or destination LISP-MNs to which it
is currently talking. The map-cache lookup is performed by doing a
longest match lookup on the destination address the IP layer put in
the first IP header. Once the new header is prepended, a route table
lookup is performed to find the interface in which to send the packet
or the default router interface is used to send the packet.
When the map-cache does not exist for a destination, the mobile node
may queue or drop the packet while it sends a Map-Request to it's
configured Map-Resolver. Once a Map-Reply is returned, the map-cache
entry stores the EID-to-RLOC state. If the RLOC state is empty in
the Map-Reply, the Map-Reply is known as a Negative Map-Reply in
which case the map-cache entry is created with a single RLOC, the
RLOC of the configured Map-Server for the LISP-MN. The Map-Server
that serves the LISP-MN also acts as a Proxy ETR (PETR) so packets
can get delivered to hosts in non-LISP sites to which the LISP-MN is
sending.
For incoming unicast packets, the LISP sub-layer simply decapsulates
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the packets and delivers to the IP layer. The loc-reach-bits can be
processed by the LISP sub-layer. Specifically, the source EID from
the packet is looked up in the map-cache and if the loc-reach-bits
settings have changed, store the loc-reach-bits from the packet and
note which RLOCs for the map-cache entry should not be used.
In terms of the LISP-MN detecting which RLOCs from each stored map-
cache entry is reachable, it can use any of the Locator Reachability
Algorithms from [LISP].
A background task that runs off a timer should be run so the LISP-MN
can send periodic Map-Register messages to the Map-Server. The Map-
Register message should also be triggered when the LISP-MN detects a
change in IP address for a given interface. The LISP-MN should send
Map-Registers to the same Map-Register out each of it's operational
links. This will provide for robustness on radio links with which
the mobile node is associated.
A LISP-MN receives a Map-Request when it has Map-Registered to a Map-
Server with the Proxy-bit set to 0. This means that the LISP-MN
wishes to send authoritative Map-Replies for Map-Requests that are
targeted at the LISP-MN. If the Proxy-bit is set when the LISP-MN
registers, then the Map-Server will send non-authoritative Map-
Replies on behalf of the LISP-MN. In this case, the Map-Server never
encapsulates Map-Requests to the LISP-MN. The LISP-MN can save
resources by not receiving Map-Requests (note that the LISP-MN will
receive SMRs which have the same format as Map-Requests).
To summarize, a LISP sub-layer should implement:
o Encapsulating and decapsulating data packets.
o Sending and receiving of Map-Request control messages.
o Receiving and optionally sending Map-Replies.
o Sending Map-Register messages periodically.
The key point about the LISP sub-layer is that no other components in
the protocol stack need changing; just the insertion of this sub-
layer between the IP layer and the interface layer-2 encapsulation/
decapsulation layer.
13. Security Considerations
Security for the LISP-MN design builds upon the security fundamentals
found in LISP [LISP] for data-plane security and the LISP Map Server
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[LISP-MS] registration security. Security issues unique to the
LISP-MN design are considered below.
13.1. Proxy ETR Hijacking
The Proxy ETR (or PETR) that a LISP-MN uses as its destination for
non-LISP traffic must use the security association used by the
registration process outlined in Section 5.2 and explained in detail
in the LISP-MS specification [LISP-MS]. These measures prevent third
party injection of LISP encapsulated traffic into a Proxy ETR.
Importantly, a PETR must not decapsulate packets from non-registered
RLOCs.
13.2. LISP Mobile Node using an EID as its RLOC
For LISP packets to be sent to a LISP-MN which has an EID assigned to
it as an RLOC as described in Section 9.1), the LISP site must allow
for incoming and outgoing LISP data packets. Firewalls and stateless
packet filtering mechanisms must be configured to allow UDP port 4341
and UDP port 4342 packets.
14. Acknowledgments
Albert Cabellos, Noel Chiappa, Pierre Francois, Michael Menth, Andrew
Partan, Chris White and John Zwiebel provided insightful comments on
the mobile node concept and on this document. A special thanks goes
to Mary Nickum for her attention to detail and effort in editing
early versions of this document.
15. IANA Considerations
This document creates no new requirements on IANA namespaces
[RFC5226].
16. References16.1. Normative References
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
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